Flow control device for production tubing

ABSTRACT

An apparatus for controlling the flow of reservoir fluids into production tubing comprises abase pipe, a screen, a nozzle being adapted to receive filtered reservoir fluids passing through the screen, and a diverter adapted to receive the fluids from the nozzle and to divert such fluids into a port extending into the base pipe. The nozzle includes at least one converging-diverging section, whereby pressure of the fluids passing there-through is reduced.

FIELD OF THE DESCRIPTION

The present description relates to flow control devices used for controlling flow into or out of pipes, in particular pipes used in oil and gas wells.

BACKGROUND

Hydrocarbon reservoirs, such as oil and/or gas reservoirs, are generally accessed by wells that are drilled into the subterranean reservoir and the hydrocarbon materials are then brought to the surface through production tubing. Production tubing consists of a plurality of pope section that connected together and inserted into the well. The well may be cased or uncased. The production tubing may also include various other tools that connected between the pipe sections, such as packers etc.

The wellbore may be vertical or horizontal or at any angle there-between. In some cases, where the hydrocarbons comprises a highly viscous material, such as heavy oil and the like, steam, gas or other fluids may be injected into one or more sections of the reservoir to stimulate the flow of hydrocarbons into the wellbore.

Steam Assisted Gravity Drainage, “SAGD”, is one example of a process that is used to stimulate the flow of highly viscous oil. In a SAGD operation, two wells, typically horizontal wells, are drilled within a reservoir. The wells comprise an upper, steam injection well, and a lower production well. In operation, steam is injected into the injection well to heat and reduce the viscosity of the hydrocarbon material. After steam treatment, the hydrocarbon material, now mobilized, drains into the production well and subsequently brought to the surface by production tubing. Cyclic Steam Stimulation, “CSS”, is another example where steam is used to enhance the mobility of viscous hydrocarbon materials. In a CSS process, a single well is used to first inject steam into the reservoir through tubing, generally production tubing. Thereafter, the heat from the steam is allowed to be absorbed into the reservoir (a stage referred to as “shut in” or “soaking”), during which the viscosity of the hydrocarbon material is reduced. Following such stage, the hydrocarbons are produced in a production stage.

Production tubing used in wellbores typically includes a number of coaxial segments, or tubulars, that are connected together. Various tools may also be provided along the length of the tubing, wherein such tools comprise a base pipe with one or more apertures, or ports, provided along its length. The ports provide a means for the inflow of hydrocarbon materials from the reservoir into the pipe and thus into the production tubing. The ports may also provide a means for the outflow, or injection of steam and/or other viscosity reducing agents from the production tubing into the reservoir. The segments having ports are also often provided with one or more filtering devices, such as wire screens, or wire-wrap screens, which serve to filter the hydrocarbon materials being produced and thereby prevent or mitigate against sand and other solid debris in the well from entering the base pipe and therefore the production tubing.

In view of the length of production tubing (which may be in the range of several thousand meters), steps must often be taken to ensure that the production rates along its length are near constant. This is to avoid preferential production from one region of a reservoir, which may result in another region not be being produced. In addition, in cases where regions of the reservoir are under high pressure, the pressure of the produced fluids may result in damage to the tubing material. Similarly, in situations where steam or another fluid is being injected, it is important to ensure that the injection is accomplished evenly so as to avoid preferential stimulation of only certain regions of the reservoir.

Various devices have been proposed for controlling the rates of production and/or injection between tubing and a reservoir. In some cases, a device such as a flow restrictor or similar nozzle is associated with the base pipe to impede the flow of fluids flowing into or from the pipe. Examples of such flow control device are described in the following references: U.S. Pat. Nos. 9,518,455; 9,638,000; 9,027,642; 7,419,002; 8,689,883; 9,249,649, US 2017/0058655 and US 2009/0078428.

There exists a need for an improved flow control means to control the flow of reservoir fluids entering into production tubing.

SUMMARY OF THE DESCRIPTION

In one aspect, there is provided an apparatus for regulating the flow of reservoir fluid entering into production tubing provided in a wellbore. In particular, the apparatus reduces or attenuates the pressure of the fluid entering into the production tubing.

In one aspect, there is provided an apparatus for controlling flow of fluids from a subterranean reservoir into production tubing provided in a well in the reservoir, the apparatus comprising:

-   -   a base pipe, adapted to be connected to the production tubing,         the base pipe having a first end and a second end and at least         one port extending through the wall thereof for conducting         reservoir fluids into the base pipe;     -   a screen for filtering reservoir fluids entering the port, the         screen being provided on the outer surface of the base pipe;     -   a nozzle provided between the screen and the port, the nozzle         having a nozzle channel for receiving fluids filtered by the         screen, the nozzle channel extending between an inlet and an         outlet of the nozzle;     -   the nozzle channel having a throat provided downstream of the         nozzle inlet and a diverging section downstream of the throat,         whereby the nozzle channel is provided with a         converging-diverging profile; and,     -   a diverter provided adjacent the nozzle outlet for diverting         fluid exiting the nozzle into the port, the diverter having a         diverter channel extending between an inlet, adapted to receive         fluids exiting the nozzle outlet, and an outlet fluidly         connected to the port on the base pipe, whereby fluids exiting         the diverter enter the port.

In another aspect, there is provided an apparatus for controlling flow of fluids from a subterranean reservoir into production tubing provided in a well in the reservoir, the apparatus comprising:

-   -   a base pipe, adapted to be connected to the production tubing,         the base pipe having a first end and a second end and at least         one port extending through the wall thereof for conducting         reservoir fluids into the base pipe;     -   a screen for filtering reservoir fluids entering the port, the         screen being provided on the outer surface of the base pipe;         and,     -   a nozzle provided between the screen and the port, the nozzle         having a nozzle channel for receiving fluids filtered by the         screen, the nozzle channel extending between an inlet and an         outlet of the nozzle;     -   the nozzle channel having: a throat provided downstream of the         nozzle inlet; a diverging section downstream of the throat; and,         an outlet section downstream of the diverging section; the         outlet section extending to the nozzle outlet and having a         constant cross-sectional area.

BRIEF DESCRIPTION OF THE FIGURES

The features of certain embodiments will become more apparent in the following detailed description in which reference is made to the appended figures wherein:

FIG. 1 is a partial cross-sectional view of a flow control apparatus according to an aspect of the description.

FIG. 2 is an enlarged section of the apparatus of FIG. 1.

FIG. 3 is an end view of a nozzle according to an aspect of the description.

FIG. 4 is a side cross-sectional view of the nozzle of FIG. 3 taken along line A-A.

FIG. 5 is a side perspective view of the nozzle of FIGS. 3 and 4.

FIG. 6 is another cross-sectional view of the nozzle shown in FIGS. 3 and 4.

FIG. 7 is a top front perspective view of a diverter according to an aspect of the description.

FIG. 8 is a top view of the diverter of FIG. 7.

FIG. 9 is a rear view of the diverter of FIG. 7.

FIG. 10 is a side view of the diverter of FIG. 7.

FIG. 11 is a side cross-sectional view of a diverter according to another aspect of the description.

DETAILED DESCRIPTION

As used herein, the terms “nozzle” or “nozzle insert” will be understood to mean a device that controls the flow of a fluid flowing there-through. In one example, the nozzle described herein serves to control the flow of a fluid through a port in a pipe in at least one direction.

The term “hydrocarbons” refers to hydrocarbon compounds that are found in subterranean reservoirs. Examples of hydrocarbons include oil and gas.

The term “wellbore” refers to a bore drilled into a subterranean formation, such as a formation containing hydrocarbons.

The term “wellbore fluids” refers to hydrocarbons and other materials contained in a reservoir that are capable of entering into a wellbore.

The terms “pipe” or “base pipe” refer to a section of pipe, or other such tubular member. The base pipe is generally provided with one or more ports or slots along its length to allow for flow of fluids there-through.

The term “production” refers to the process of producing wellbore fluids.

The term “production tubing” refers to a series of pipes, or tubulars, connected together and extending through a wellbore from the surface into the reservoir.

The terms “screen”, “sand screen”, “wire screen”, or “wire-wrap screen”, as used herein, refer to known filtering or screening devices that are used to inhibit or prevent sand or other solid material from the reservoir from flowing into the pipe. Such screens may include wire wrap screens, precision punched screens, premium screens or any other screen that is provided on a base pipe to filter fluids and create an annular flow channel. The present description is not limited to any particular screen described herein.

The terms “comprise”, “comprises”, “comprised” or “comprising” may be used in the present description. As used herein (including the specification and/or the claims), these terms are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not as precluding the presence of one or more other feature, integer, step, component or a group thereof as would be apparent to persons having ordinary skill in the relevant art.

In the present description, the terms “top”, “bottom”, “front” and “rear” may be used. It will be understood that the use of such terms is purely for the purpose of facilitating the description of the embodiments described herein. These terms are not intended to limit the orientation or placement of the described elements or structures.

FIG. 1 illustrates a partial side cross sectional view of an embodiment of an apparatus that may be used for controlling the production rate from a reservoir. FIG. 2 illustrates an enlarged view of a portion of the apparatus of FIG. 1. The apparatus 10 comprises a base pipe 12 that is adapted to be connected to members of a tubing string, or production string. Such members are commonly referred to as tubulars. As with tubulars, the base pipe 12 would be provided with a threaded male portion, or “pin”, at one end a threaded female portion, or “box”, at the opposite end. As known in the art, each of such ends (not shown in FIG. 1) would be adapted to connect to a corresponding end of an adjacent tubular. It will be understood that in some instances other tools or components (such as packers etc.) may be provided on the production string and that such other components may be positioned adjacent to the base pipe 12.

The apparatus 10 also includes a filter or screen as known in the art. In the example illustrated in the accompanying figures, the filter is a wire screen 14, which, as known in the art, comprises a plurality of circumferentially spaced rib wires 16 that extend longitudinally over a portion of the base pipe 12. The wire screen 14 also includes a circumferential wire wrap 18 provided over the ribs 16. As would be known to persons skilled in the art, the wire screen 14 results in a plurality of longitudinally extending channels between adjacent rib wires 16. In addition, each winding of the wire wrap 18 is spaced by a given distance, thereby allowing fluids to pass there-through but filtering out solid material having a diameter greater than the spacing between the wires. As indicated above, although a wire screen, or wire-wrap screen, is described herein, it will be understood that various other filtering devices may be used with the apparatus described herein. The present description is not intended to be limited to any particular filtering device. For example, the filtering device may comprise a slotted liner or the like, which serves to filter fluids and create an annular flow channel whereby fluids are flowed to one or more ports on a base pipe. Although the present description will refer to a wire screen for convenience, it will be understood that this is not intended to be limiting in any way.

As known in the art, the wire screen 14 is secured to the base pipe 12 and this may be accomplished by various means. In one aspect, the wire screen 14 is secured to the base pipe 12 by means of circumferential collars provided on each end of the screen. As shown in FIG. 1, a first collar 20 is provided on a first end of the wire screen 14 and a second collar 22 is provided on a second end of the wire screen. The collars 20 and 22 may be secured to the base pipe 12 by any known means. In one aspect, the collars may be welded to the base pipe 12.

The apparatus includes an annular space 24 adjacent the second end of the wire screen 14, as described further below.

The base pipe 12 includes at least one port or aperture 26 providing fluid communication to the lumen of the pipe. The pipe 12 is preferably also provided with a recess 13 on the outer surface of thereof. In a preferred embodiment, the recess 13 is provided around the port 24. The recess 13 is sized to receive and retain a diverter 28. As shown in FIGS. 1 and 2, the diverter 28 includes an outlet 30 that is fluidly connected to the port 24, whereby fluids exiting the outlet 30 of the diverter 28 enter into the base pipe 12 through the port 24. As illustrated in the accompanying figures, the diverter outlet 30 is provided on a surface of the diverter 28 that contacts the outer surface of the base pipe 12. As shown in FIGS. 1 and 2, when the diverter is provided on the base pipe 12, the diverter outlet 30 is adapted to overlie the port 24 so as to be fluidly connected thereto. Further description of the diverter 28 is provided below.

The apparatus further includes a nozzle 32 positioned between the annular space 24 adjacent the wire screen 14 and the diverter 28. As more clearly shown in FIG. 2, the nozzle includes an inlet 34 adjacent the annular space 24, and an outlet 36 adjacent to, and preferably connected to an inlet 38 of the diverter. As would be understood, when the apparatus is in use, fluids from the reservoir pass through the wire screen and enter into the annular space 24 and then enter into the inlet 34 of the nozzle 32. The fluids pass through the nozzle 32, exiting the outlet 36 thereof and enter into the inlet 38 of the diverter 28. The fluids then pass through the outlet 36 of the diverter 28 entering the port 26 and consequently into the lumen of the base pipe 12. As noted above, the base pipe 12 is connected to and forms part of the production string and, as such, the fluids entering through the port 26 are produced at the surface.

As noted above, the base pipe 12 shown in FIGS. 1 and 2 includes a recess 13 for receiving and retaining the diverter 28. It will be understood that in other embodiments a similar recess may be provided on the base pipe 12 for receiving and retaining the nozzle 32.

The nozzle 32 will now be discussed in further detail. As shown more clearly in FIG. 2, the nozzle includes a channel 40 extending there-through, from the inlet 34 to the outlet 36. The channel 40 includes a throat 42 adjacent to and downstream from the inlet 34. Further downstream of the throat 42, the channel 40 includes a diverging region 44 having an increasing cross-sectional area in the direction extending between the throat 42 and the outlet 36. In one aspect, the diverging region 44 extends to the outlet 36. However, in the embodiment illustrated in FIGS. 1 and 2, the diverging region 44 extends only partway along the length of the channel 40, terminating in a region of constant cross-sectional area 46. As will be understood, the combination of the throat 42 and the diverging region 44 result in the nozzle having a convergent-divergent channel, such as a Venturi tube. As would be understood by persons skilled in the art, as fluids entering the inlet 34 of the nozzle are passed through the converging throat 42 and the diverging region 44, the pressure of the fluids is dissipated while its velocity is increased.

As illustrated in FIGS. 1 and 2, the fluid passing through the channel 40 of the nozzle 32 generally maintain a flow path that extends longitudinally along the base pipe 12. Once the fluid passes through the nozzle 32, it enters the diverter 28, which serves to divert the flow path of the fluid from one extending longitudinally to one extending generally radially with respect to the base pipe 12. As shown in the figures, the diverter 28 includes a channel 48 extending between the inlet 38 and outlet 30 thereof. The channel 48 includes a bend resulting in the inlet 38 and outlet 30 being generally orthogonal to each other. As will be understood, fluid entering the nozzle 32 (as described above), subsequently enters the diverter 28 and then into the pipe 12 through the port 26.

As illustrated in FIGS. 1 and 2, the second collar 22, is sized so as to overlap the wire screen 14 as well as the nozzle 32 and the diverter 28. The collar 22 may be adapted to receive and retain the nozzle 32 and/or the diverter 28 in position over the pipe when the collar 22 is secured thereto. As more clearly shown in FIG. 2, the collar 22 is provided with a recess 29 for the purpose of receiving and retaining the diverter 28. It will be understood that in another embodiment the collar 22 may include a further recess for receiving and retaining the nozzle 32.

FIGS. 3 to 6 illustrate the nozzle of the present description in isolation, wherein the same reference numerals are used to identify similar elements. As shown, the nozzle 32 comprises a body having a generally monolithic structure. The inlet 34, in the same manner as described above, comprises an opening into a channel 40. The channel 40 includes a throat 42 located downstream from the inlet 34. As discussed above, the throat 42 serves as a choke point or point of convergence for fluid flowing there-through. In one embodiment, the throat 42 comprises a smooth curved wall, as illustrated, at both the inlet end and the outlet end thereof. However, in other embodiments, the wall of the throat and the surrounding region of the channel 40 may be provided with a stepped cross-sectional appearance. As discussed above, the throat 42 serves to dissipate pressure of the fluid flowing through the channel 40. As will be understood, by providing the throat with a stepped wall, or other such geometry that is not smooth, the throat will be able to dissipate even further pressure of the flowing fluid.

Downstream from the throat 42, the channel 40 is provided with a region of increasing diameter, or a diverging region 44, and an outlet 36. In one embodiment, as illustrated, the channel 40 includes a region of constant cross-sectional area, 46, between the diverging region 44 and the outlet 36. In the illustrated embodiment, the diverging region 44 and, where present, the region of constant diameter 46 are provided with smooth walls. However, as with the throat 42, the walls of these regions may also, together or independently, be provided with a stepped or otherwise non-smooth surface to enhance the dissipation of pressure of the fluid flowing there-through.

FIG. 5 illustrates a side view of the nozzle 32 while FIG. 6 illustrates a perspective cross-sectional view thereof.

FIGS. 7 to 10 illustrate an embodiment of the diverter 28. As shown, the diverter comprises a monolithic element that is positioned within the recess 13 in the base pipe 12 as described above. As also discussed above, the diverter 28 has an opening or inlet 38 that, when in use, receives fluids exiting the nozzle 32. In one aspect, the inlet 38 of the diverter 28 is provided with a recess 39 for receiving the end of the nozzle 32 having the outlet 36. As would be understood, the recess 39 facilitates the flow of fluid from the nozzle outlet 36 into the diverter inlet 38. The diverter 28 also includes an outlet 30 that, when in use, diverts the fluid into a port on the pipe as described above. As mentioned above, the diverter 28 includes an internal channel 48 extending between the inlet 38 and outlet 30 and through which the fluid flows. The channel 48 is shown in phantom in FIGS. 8 to 10. In the embodiment shown in FIGS. 7 to 10, the diverter channel 48 comprises an arcuate elbow extending between the inlet 38 and outlet 30. The channel 48 may, in one aspect, comprise a smooth, curved surface shown in phantom at 50 in FIGS. 8 to 10. In another embodiment, the surface of the diverter channel 48 may have a stepped or other such non-smooth surface.

As also shown, the outlet 30 has a larger cross-sectional area than the inlet 38. As such, the diverter channel 48 is preferably provided with an increasing cross-sectional area extending in the direction from the inlet 38 to the outlet 30. In this way, the diverter serves to further reduce the pressure of the fluid flowing there-through while diverting the fluid into the base pipe.

FIG. 11 illustrates another embodiment of the diverter wherein like elements are identified with like reference numerals but with the letter “a” added for clarity for elements that are variants. As shown in FIG. 11, the diverter 28 a includes an inlet 38 a and an outlet 30 a. Although not shown, the diverter inlet 38 a may include a recess (such as that shown at 39 in FIG. 7) for receiving the outlet end of the nozzle. In this embodiment, the diverter channel 48 a comprises two sections, namely and inlet section 52 and an outlet section 54, that are connected at an elbow 56. As illustrated in FIG. 11, the inlet section 52 of the channel 48 a comprises a generally linear conduit that is aligned with the channel of the nozzle. As described above, the inlet section 52 of the channel 48 a receives fluid exiting the nozzle 32. In the embodiment shown in FIG. 11, the inlet section 52 of the channel 48 a is provided with a generally constant cross-sectional area. As such, the inlet section 52 may be cylindrical in shape. The outlet section 54 of the diverter channel 48 a, however, comprises a diverging channel, having an increasing cross-sectional area in the direction towards the outlet 30 a. In the embodiment shown, the outlet section 54 has a generally conical, or frustoconical shape extending from the elbow 56 to the outlet 30. As also shown, the outlet section 54 is provided at an angle with respect to the inlet section 52. As will be understood, the diverging nature of the channel 48 a, in particular the outlet section 54 thereof, further serves to reduce the pressure of the fluid passing there-through.

Although the above description includes reference to certain specific embodiments, various modifications thereof will be apparent to those skilled in the art. Any examples provided herein are included solely for the purpose of illustration and are not intended to be limiting in any way. Any drawings provided herein are solely for the purpose of illustrating various aspects of the description and are not intended to be drawn to scale or to be limiting in any way. The scope of the claims appended hereto should not be limited by the preferred embodiments set forth in the above description, but should be given the broadest interpretation consistent with the present specification as a whole. The disclosures of all prior art recited herein are incorporated herein by reference in their entirety. 

1. An apparatus for controlling flow of fluids from a subterranean reservoir into production tubing provided in a well in the reservoir, the apparatus comprising: a base pipe, adapted to be connected to the production tubing, the base pipe having a first end and a second end and at least one port extending through the wall thereof for conducting reservoir fluids into the base pipe; a screen for filtering reservoir fluids entering the port, the screen being provided on the outer surface of the base pipe; a nozzle provided between the screen and the port, the nozzle having a nozzle channel for receiving fluids filtered by the screen, the nozzle channel extending between an inlet and an outlet of the nozzle; the nozzle channel having a throat provided downstream of the nozzle inlet and a diverging section downstream of the throat, whereby the nozzle channel is provided with a converging-diverging profile; and, a diverter provided adjacent the nozzle outlet for diverting fluid exiting the nozzle into the port, the diverter having a diverter channel extending between an inlet, adapted to receive fluids exiting the nozzle outlet, and an outlet fluidly connected to the port on the base pipe, whereby fluids exiting the diverter enter the port.
 2. The apparatus of claim 1, wherein the nozzle channel includes an outlet section downstream of the diverging section, the outlet section having a constant cross-sectional area extending to the nozzle outlet.
 3. The apparatus of claim 1, wherein the nozzle channel is generally aligned with the longitudinal axis of the base pipe.
 4. The apparatus of claim 1, wherein the diverter channel comprises an inlet portion aligned with the nozzle channel and an outlet portion aligned with the port.
 5. The apparatus of claim 4, wherein the diverter channel comprises an elbow extending from the diverter inlet to the diverter outlet.
 6. The apparatus of claim 5, wherein the diverter outlet has a larger cross-sectional area than the diverter inlet and wherein the cross-sectional area of the diverter channel increases in a direction from the diverter inlet to the diverter outlet.
 7. The apparatus of claim 4, wherein the diverter channel comprises an inlet portion and an outlet portion, the inlet portion and outlet portion being joined at an elbow, the inlet portion extending from the diverter inlet and being generally aligned with the nozzle channel, the outlet portion extending from the elbow to the diverter outlet.
 8. The apparatus of claim 7, wherein the inlet portion of the diverter channel has a constant cross-sectional area and wherein the cross-sectional area of the outlet portion increases in the direction from the elbow to the diverter outlet.
 9. The apparatus of claim 7, wherein the outlet portion of the diverter channel is angled with respect to the inlet portion of the diverter channel.
 10. The apparatus of claim 7, wherein the outlet portion of the diverter channel has a conical or frustoconical shape.
 11. The apparatus of claim 1, wherein the base pipe includes a first recess to receive the diverter.
 12. The apparatus of claim 1, wherein the base pipe includes a second recess to receive the nozzle.
 13. The apparatus of claim 1, wherein the screen is retained on the base pipe with one or more collars and wherein at least one of said collars overlaps the nozzle and the diverter.
 14. The apparatus of claim 13, wherein the at least one collar includes a first recess to receive the diverter.
 15. The apparatus of claim 13, wherein the at least one collar includes a second recess to receive the nozzle.
 16. The apparatus of claim 1, wherein the nozzle channel comprises a smooth surface.
 17. The apparatus of claim 1, wherein the nozzle channel comprises a stepped surface.
 18. The apparatus of claim 1, wherein the diverter channel comprises a smooth surface.
 19. The apparatus of claim 1, wherein the diverter channel comprises a stepped surface.
 20. The apparatus of claim 1, wherein the diverter includes a recess adapted to receive the nozzle outlet.
 21. The apparatus of claim 1, wherein the diverter and the nozzle are separate elements.
 22. An apparatus for controlling flow of fluids from a subterranean reservoir into production tubing provided in a well in the reservoir, the apparatus comprising: a base pipe, adapted to be connected to the production tubing, the base pipe having a first end and a second end and at least one port extending through the wall thereof for conducting reservoir fluids into the base pipe; a screen for filtering reservoir fluids entering the port, the screen being provided on the outer surface of the base pipe; and, a nozzle provided between the screen and the port, the nozzle having a nozzle channel for receiving fluids filtered by the screen, the nozzle channel extending between an inlet and an outlet of the nozzle; the nozzle channel having: a throat provided downstream of the nozzle inlet; a diverging section downstream of the throat; and, an outlet section downstream of the diverging section; the outlet section extending to the nozzle outlet and having a constant cross-sectional area.
 23. The apparatus of claim 22, wherein the nozzle channel is generally aligned with the longitudinal axis of the base pipe.
 24. The apparatus of claim 22, wherein the base pipe includes a recess to receive the nozzle.
 25. The apparatus of claim 22, wherein the screen is retained on the base pipe with one or more collars and wherein at least one of the collars overlaps the nozzle.
 26. The apparatus of claim 25, wherein the at least one collar includes a recess to receive the nozzle.
 27. The apparatus of claim 22, wherein the nozzle channel comprises a smooth surface.
 28. The apparatus of claim 22, wherein the nozzle channel comprises a stepped surface. 